The present invention relates to a new and improved process for the production of crystalline alkali metal aluminum phosphates. More particularly, the present invention relates to a process for the production of crystalline alkali metal aluminum phosphates, referred to as "SALP," that are represented by the formula: EQU Na.sub.a K.sub.b H.sub.c Al.sub.d (PO.sub.4).sub.e.xH.sub.2 O,
wherein a has a value of from 0.5 to 1.5 and preferably of from 0.8 to 1.1, b has a value of from 0 to 0.5, c has a value of from 12 to 16, d has a value of from 2.7 to 3.3, e has a value of from 7 to 9 and x has a value of from 3 to 6.
SALP is used as a bakery rising acid in finished pastry, deep-cooled pastry, baking mixtures and self-rising flours, among other compositions. The properties and uses of SALP are described in detail in U.S. Pat. Nos. 2,550,491, 3,109,738 and 3,501,314, and in West German Offenlegungsschriften No. 27 30 226 and No. 27 49 581.
SALP is usually produced from phosphoric acid, sodium hydroxide or sodium carbonate, and aluminum hydroxide. Depending upon the water content of the reactants, a reaction mixture is obtained, as a viscous supersaturated solution, which contains about 5 to 44% of free water. For purposes of this description, the phrase "free water" is understood to mean the total water content of the reaction mixture, less the content of water of crystallization of the product.
Dry SALP is obtained from this viscous solution either by precipitation with a methanol/water mixture (U.S. Pat. No. 2,550,490); by evaporation of the water discontinuously (U.S. Pat. No. 2,957,750) or continuously (West German Auslegeschrift No. 11 91 795; U.S. Pat. No. 3,311,448) operating kneading apparatus; by spray drying (U.S. Pat. No. 2,995,421) or by precipitation with dilute phosphoric acid (European patent application No. 0 031 803).
It is a common feature of all precipitation processes (as exemplified in U.S. Pat. No. 2,550,490 and European patent application No. 0 031 803) that the yields obtained are generally unsatisfactory, ranging from about 60 to 70%. The amounts of waste water produced in these processes also give rise to problems. Furthermore, a precipitation with methanol is considered infeasible because of the unavoidable contamination of the product with toxic methanol.
Spray drying (U.S. Pat. No. 2,995,421) gives an amorphous product which is very hygroscopic. But SALP must be distinctly crystalline and not very hygroscopic to be useful as a bakery rising acid.
West German Auslegeschrift No. 11 91 795 (corresponding to U.S. Pat. No. 3,311,448) discloses a process whereby SALP is produced discontinuously in the form of a viscous supersaturated solution, which is then passed continuously through an intermediate container to a crystallization apparatus (kneader). The reaction solution is there applied to an existing bed of crystalline SALP.
In this and the other drying processes previously mentioned, the reaction mixture passes through a phase of extremely high viscosity shortly before crystallization, resulting in high stresses on the working-up apparatus. This problem is evident from a statement in West German patent specification No. 11 91 795 that the
viscosities which arise in the production of acidic sodium aluminum phosphates can only be quantitatively suppressed with difficulty since they lie outside of the range of conventional viscosimeters, perhaps in the range of from 100,000 to 200,000 cP. The viscosity of the phase predent immediately before crystallization corresponds roughly to that of synthetic resin asphalts or of the non-volatile components of mineral oil.
For these processes, therefore, it is necessary to use powerful kneading devices, which are costly and consume large amounts of energy.
The poor working-up capacity of the reaction mixtures described heretofore is due to the fact that, in the prior-art processes, the free water has already been removed from the reaction mixtures before the crystallization commences. As a result, the reaction mixture is first present as a supersaturated viscous "solution." Shortly before the complete removal of the free water, crystallization takes place spontaneously. At this point, the extreme increase of the viscosity also occurs, and the product can only be kept moving with overdimensioned driving apparatus. Of course, this extreme stressing also results in increased wear.